1. Field of the Invention
The invention relates to an exhaust gas control apparatus and a control method for an exhaust gas control apparatus.
2. Description of Related Art
A nitrogen oxide (NOx) reduction catalyst that reduces NOx contained in exhaust gas from an internal combustion engine using ammonia as a reducing agent is employed as an exhaust gas control member. In an exhaust gas control apparatus having a NOx reduction catalyst, ammonia is supplied to the NOx reduction catalyst from an upstream side of the NOx reduction catalyst. A method of injecting urea water into the exhaust gas is generally employed as an ammonia supply method.
The urea water injected into the exhaust gas decomposes in the exhaust gas so as to generate ammonia. The ammonia generated from the urea water is adsorbed to the NOx reduction catalyst. A redox reaction is promoted between the ammonia adsorbed to the NOx reduction catalyst and the NOx in the exhaust gas that flows into the NOx reduction catalyst, and as a result, the NOx is removed from the exhaust gas.
A NOx purification rate of the NOx reduction catalyst depends on the amount of ammonia adsorbed to the NOx reduction catalyst. As the amount of ammonia adsorbed by the NOx reduction catalyst increases, a steadily higher NOx purification rate is obtained. There is, however, an upper limit to the amount of ammonia that can be adsorbed to the NOx reduction catalyst, and when an excessive amount of ammonia exceeding the upper limit is supplied to the NOx reduction catalyst, ammonia slip, whereby the ammonia that cannot be adsorbed to the NOx reduction catalyst flows out of the NOx reduction catalyst, becomes more likely to occur.
Hence, in an exhaust gas control apparatus having a NOx reduction catalyst, it is difficult to realize an improvement in the NOx purification rate while suppressing ammonia slip.
In response to this problem, Japanese Patent Application Publication No. 2003-293737 (JP-A-2003-293737) proposes an invention that achieves an improvement in the NOx purification rate while suppressing ammonia slip by calculating an actual ammonia adsorption amount adsorbed by a NOx reduction catalyst on the basis of an ammonia consumption amount consumed by the NOx reduction catalyst and an ammonia addition amount added by a reducing agent supply device, and controlling the ammonia addition amount on the basis of the calculated actual adsorption amount so that the amount of ammonia adsorbed to the NOx reduction catalyst reaches a target adsorption amount.
Even when the amount of ammonia adsorbed to the NOx reduction catalyst remains constant, the NOx purification rate and the likelihood of ammonia slip differ according to a distribution of the ammonia adsorption amount through the NOx reduction catalyst.
For example, immediately after the urea water has been added from the upstream side of the NOx reduction catalyst, the amount of ammonia adsorbed to an upstream side part of the NOx reduction catalyst in an exhaust gas flow direction may be greater than the amount of ammonia adsorbed to a downstream side part. A higher NOx purification rate is obtained when the ammonia adsorption amount is distributed in this manner than when the adsorbed ammonia is distributed uniformly (evenly) throughout the NOx reduction catalyst.
Once a certain amount of time has elapsed following addition of the urea water, on the other hand, the amount of ammonia adsorbed to the downstream side part of the NOx reduction catalyst in the exhaust gas flow direction may become greater than the amount of ammonia adsorbed to the upstream side part. When the ammonia adsorption amount is distributed in this manner, ammonia slip is more likely to occur than when the adsorbed ammonia is distributed uniformly throughout the NOx reduction catalyst.
In the invention described in JP-A-2003-293737, an overall ammonia adsorption amount of the NOx reduction catalyst is simply controlled to the target adsorption amount, and differences in the NOx purification rate and the likelihood of ammonia slip due to the distribution of the ammonia adsorption amount through the NOx reduction catalyst are not taken into account. It may therefore be impossible to realize sufficiently an effect of improving the NOx purification rate while suppressing ammonia slip.